As shown in FIG. 1, a conventional gear tooth sensor [1] consists of an IC (integrated circuit) 11 that includes Hall effect sensors 12 together with a single hard magnet 13. The IC supports two Hall sensors, which sense the magnetic profile of the ferrous target simultaneously, but at different points, thereby generating a differential internal analog voltage that is further processed for precise switching of the digital output signal. To achieve a high differential signal output, the two Hall probes (or sensors) are spaced at a certain distance so that one Hall sensor faces field concentrating tooth 14 and the other Hall sensor faces gap 15 in the toothed wheel. A permanent magnet mounted with one pole on the rear side of the IC produces a constant magnetic bias field.
If one Hall sensor momentarily faces a tooth while the other faces a gap between teeth, the gear tooth acts as a flux concentrator. It increases the flux density through the Hall probe and a differential signal is produced. As the gear wheel turns, the differential signal changes its polarity at the same rate of change as from the tooth to the gap. An integrated highpass filter regulates the differential signal to zero by means of a time constant that can be set with an external capacitor. In this way only those differences that changed at a minimum rate are evaluated. The output signal is not defined when in the steady state.
A GMR based gear tooth sensor has also been described in which the sensing structure is similar to traditional Hall IC based gear tooth sensor except that the two Hall probes are replaced by two GMR sensors [2]. As shown in FIGS. 2a-d, the magnetic field generated by the bias magnet is influenced by the moving gear tooth, the GMR sensors serving to detect the variation of the magnetic field component within the GMR film plane. The signal output is then generated from differential signals from two GMR sensors or a GMR bridge. Since the permanent magnet is mounted with either pole on the rear side of the GMR sensors (as in the Hall IC based gear tooth sensor) the magnetic field is essentially perpendicular to the GMR films.
If two or more permanent magnets are used to generate the bias field, it becomes possible to locate the GMR sensors within an area in which the magnetic field is zero when the sensor is opposite a gap between teeth, rising to its maximum value when opposite a tooth.
References
1. Infineon application note “Dynamic Differential Hall Effect Sensor IC TLE 4923”
2. NVE application note “Precision Gear Tooth and Encoder Sensors”
A routine search of the prior art was performed with the following references of interest being found:
In U.S. Patent Application 2006/0261801, Busch teaches four AMR elements arranged in a Wheatstone bridge to form a gear tooth sensor. U.S. Pat. No. 7,112,957 (Bicking) discloses MR sensors in a Wheatstone bridge to sense gear teeth in various positions. A permanent magnet is also disclosed.
U.S. Pat. No. 5,351,028 (Krahn) shows a gear tooth sensor using a permanent magnet and MR elements in a Wheatstone bridge. In U.S. Pat. No. 7,195,211, Kande et al. teach that a gear tooth sensor can comprise a Hall effect sensor or a magneto-restive sensor while, in U.S. Pat. No. 7,138,793, (Bailey shows that a gear tooth sensor can be a Hall effect sensor or a GMR sensor.